Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
While the disk drive is centerline tracking a data track during write/read operations, there may be repeatable disturbances induced in the servo control system, such as a repeatable runout (RRO) due to an eccentricity of the servo tracks. There may also be a repeatable disturbance due to a “written-in error” of the servo sectors. The RRO and “written-in error” may be considered fixed disturbances that remain substantially constant over the life of the disk drive. Prior art disk drives have typically learned the RRO to generate feed-forward compensation values that force the head to follow the eccentric servo tracks, as well as feedback compensation values that force the head to ignore the repeatable disturbance due to the written-in error. The feed-forward compensation values are typically learned for each zone of each disk surface, wherein each zone represents a band of servo tracks. The written-in error is typically learned for each servo track, and corresponding compensation values recorded in each servo sector of each servo track.